Products made from base webs such as bath tissues, facial tissues, paper towels, industrial wipers, food service wipers, napkins, medical pads, and other similar products are designed to include several important properties. For example, the products should have a soft feel and, for most applications, should be highly absorbent. The products should also have good stretch characteristics and should resist tearing. Further, the products should also have good strength characteristics, should be abrasion resistant, and should not deteriorate in the environment in which they are used.
In the past, many attempts have been made to enhance and increase certain physical properties of such products. Unfortunately, however, when steps are taken to increase one property of these products, other characteristics of the products may be adversely affected. For instance, the softness of nonwoven products, such as various paper products, can be increased by several different methods, such as by selecting a particular fiber type, or by reducing cellulosic fiber bonding within the product. Increasing softness according to one of the above methods, however, may adversely affect the strength of the product. Conversely, steps normally taken to increase the strength of a fibrous web typically have an adverse impact upon the softness, the stiffness or the absorbency of the web.
The present invention is directed to improvements in base webs and to improvements in processes for making the webs in a manner that optimizes the physical properties of the webs. In particular, the present invention is directed to a process for improving the tactile properties, such as softness and stiffness, of base webs without severely diminishing the strength of the webs. The present invention is also directed to a process for reducing the caliper of nonwoven webs.
As stated above, the present invention is directed to further improvements in prior art constructions and methods, which are achieved by providing a process for producing base webs, namely base webs containing pulp fibers. The process includes the step of first forming a base web. The base web can be made from various fibers and can be constructed in various ways. For instance, the base web can contain pulp fibers and/or staple fibers. Further, the base web can be formed in a wet lay process, an air forming process, or the like.
Once the base web is formed, the web is placed in between a first moving conveyor and a second moving conveyor. The first and second moving conveyors are then guided around a shear inducing roll while the base web is positioned in between the conveyors. The conveyors are sufficiently wrapped around the shear inducing roll and are placed under a sufficient amount of tension so as to create shear forces that act upon the base web. The shear forces disrupt the web increasing the softness and decreasing the stiffness of the web. Of particular advantage, it has been discovered that the softness of the web is increased without substantially reducing the strength of the web. More particularly, it has been discovered that the process shifts the normal strength-softness curve so as to create webs having unique softness and strength properties.
For some applications, it may be desirable to decrease the caliper of a web while still gaining all of the above advantages. For such a situation, it may be desirable to combine the shear inducing process with a calendering process. This system can provide additional caliper reduction of the web at nips formed using the shear inducing roll itself as a nip roll. The shear inducing roll can contact support rolls located on either side of the shear inducing roll. The conveyors can then wrap around the support roll, pass through the first nip, wrap around the shear inducing roll, and pass through the second nip before wrapping around the second support roll. In one particular embodiment, the conveyors can wrap around the shear inducing roll in an amount greater than 180xc2x0.
In one embodiment of a shear inducing/nip roll combination system, the shear inducing roll can be fixed in only the cross machine, or axial direction of the roll, and free to xe2x80x98floatxe2x80x99 in other directions. This can allow the tension of the conveyors passing over the shear inducing roll to pull the shear inducing roll against the support rolls. In this manner, the tension placed on the conveyors can control the nip pressures. The axis of the shear inducing roll can be placed either above or below the plane defined by the axes of the support rolls, with the support rolls close enough to each other that the shear inducing roll cannot pass between them. In general, the support rolls can have diameters greater than the shear inducing roll, for example, greater than 20 inches, but they need not have diameters equal to each other.
The shear inducing roll can rotate or can be a stationary device. The shear inducing roll can have any diameter that permits the introduction of shear forces in the web. For example, the roll can have a diameter of up to 20 inches or larger. For most applications, however, the shear inducing roll can have a small effective diameter, such as less than about 10 inches, particularly less than about 7 inches and more particularly from about 2 inches to about 6 inches. For most applications, the conveyors should be wrapped around the shear inducing roll at least 40xc2x0, and particularly from about 80xc2x0 to about 270xc2x0. Further, the amount of tension placed upon the conveyors when wrapped around the shear inducing roll should be at least 5 pounds per linear inch and particularly from about 10 pounds per linear inch to about 50 pounds per linear inch.
In one embodiment, the shear inducing roll can be supported by an air film on a bearing. The bearing can be on a stiff, stationary beam comprised of one or more gas chambers which provide air through the bearing to support the roll. If more than one chamber is in the beam, each chamber can be supplied by separately controlled pressure regulation in order to keep the shear inducing roll centered on the bearing. Such an embodiment can allow for a very small diameter shear inducing roll, such as less than ten inches, and can prevent deflection of the roll across the web due to the support of the bearing beneath the roll.
In another possible embodiment, the shear inducing roll can be in the form of a stiff, stationary shoe having a convex outer edge. In addition, the shoe can have an impermeable polymer belt surrounding it which can be free to rotate around the shoe. The conveyors can pass over the convex edge of the shoe while in contact with the rotating polymer belt. Such a system can allow for a small effective diameter for inducing shear, such as 10 inches or less, and also can prevent roll deflection across the shear inducing roll.
When guided around the shear inducing roll, the base web should have a moisture content of less than about 10%, particularly less than about 5% and more particularly less than about 2%.
As described above, various types of base webs can be processed according to the present invention. For example, in one embodiment, the base web can be a stratified web including a middle layer positioned between a first outer layer and a second outer layer. In one embodiment, the outer layers can have a tensile strength greater than the middle layer. For example, the outer layers can be made from softwood fibers, while the middle layer can be made from hardwood fibers.
Alternatively, the middle layer can have a tensile strength greater than the outer layers. It has been discovered by the present inventors that various unique products can be formed when using stratified base webs as described above.
Base webs processed according to the present invention can have various applications and uses. For instance, the webs can be used and incorporated into bath tissues, facial tissues, paper towels, industrial wipers, food service wipers, napkins, medical pads, diapers, feminine hygiene products, and other similar products.
Other features and aspects of the present invention are discussed in greater detail below.